Cerium based composite oxides in the form of fine spherical particles have been in use as ceramic materials utilizing cerium based composite oxides. Such fine spherical particles are conventionally known to be prepared by precipitation or hydrothermal synthesis. An example of the precipitation method reported is a method wherein precipitate of cerium based composite salt is obtained by coprecipitation, separated by filtration, and calcined to obtain a oxide powder material (JP-8-169713-A). An example of the hydrothermal synthesis proposed is a method wherein precipitate of a cerium based composite salt is processed at high temperature under high pressure to synthesize fine spherical particles.
As far as the primary particles are concerned, both the coprecipitation and hydrothermal synthesis mentioned above are capable of providing fine spherical powders. However, the fineness of the powders causes difficulties in washing and filtration in the powder production process, resulting in remarkable aggregation of the oxide powders and a disadvantageously broad particle size distribution. Further, the hydrothermal synthesis involves processing at high temperature under high pressure in a pressure vessel such as an autoclave, so that the industrial productivity is low, and the production cost is high. In addition, when the powders with a broad particle size distribution are directly used as ceramic materials, the powders cause problems in processibility and sinterability, such as cracks in molding or sintering, and insufficient density of sintered products. Thus such powders with a broad particle size distribution requires pre-treatments such as grinding, dispersing, or sieving, before use as ceramic materials, which increases the process steps in the ceramics production and adds to the production cost.
On the other hand, JP-8-169713-A discloses a highly sinterable cerium based composite oxide that provides a sintered product having a relative sintering density of 98.6% at a sintering temperature of 1450° C. However, the raw material powders have a broad particle size distribution and include coarse particles, so that the powders cannot be molded homogeneously and densely. This causes generation of voids in the molded body, which cannot be eliminated during sintering. Thus densely sintered products having a relative sintering density of not lower than 99% have not been obtained.